This invention relates generally to the field of telephony, and more particularly to an improved maintenance termination unit serving to open a subscriber circuit at the demarcation point, normally the point where subscriber equipment is connected, so that the central office can conduct necessary tests to determine the condition of that part of the line for which it is responsible. Devices of this general type are known in the art, and the invention lies in specific details of structure which permit both lower manufacturing costs, a smaller casing or housing, and, because of simplicity, improved reliability in operation.
Subscriber ownership of on-premises telephone equipment is now commonplace. With it, the problem for joint responsibility on the part of the telephone company and the subscriber for the maintenance of their respective sections of the subscriber line emanating from both sides of a demarcation point, usually the entrance terminal, arises. It is readily appreciated that the major portion of the line is the property and the responsibility of the telephone company, and as a practical matter, the subscriber performs little, if any, periodic tests of his equipment. The bulk of the tests initiated by the telephone company require the opening of the line to the subscriber, and the effective closing of a loop off the subscriber premises as a result of passage of predetermined voltages to a maintenance termination unit located at the demarcation point which includes a voltage responsive switch. Once actuated, subsequent tests are conducted at alternating current voltages compatible with the holding of the switch in open condition. When the tests are completed, the normal direct current voltage is restored, and the maintenance termination unit returns to normal condition resulting in re-establishment of communication with the subscriber-owned equipment.
Prior art structure for performing the above described functions has been relatively complicated and extensive, requiring the presence of a diode bridge and an integrated circuit having a Zener diode, several resistors and a pair of transistors. This unit requires a housing, the largest diameter of which is approximately four inches. Thus, the unit requires a substantial amount of space within the building entrance terminal.